The present invention relates to automotive vehicles, and more particularly to powertrain control of automotive vehicles.
Vehicles typically use a torque converter to transfer torque between an engine and an automatic transmission. To improve fuel economy, torque converters typically have a lock-up clutch. Under certain operating conditions, the clutch is engaged to provide a direct coupling between the engine and transmission. Certain types of torque converters use two hydraulic circuits to provide the ability to lock up when transferring a certain amount of positive torque. Such ability allows for lock-ups during cruising.
The inventors herein have recognized that it is desirable to lock the torque converter when transmitting large negative torque values to increase engine braking, reduce brake wear, improve drivability, and increase fuel economy. The inventors herein have also recognized a potential disadvantage with the above approach when using a torque converter of the type described above. In particular, the inventors have recognized that such a torque converter is difficult to lock up when the powertrain is transmitting large negative torque values. Specifically, when transmitting large negative torques, the hydraulic pressure available for applying the clutch is low and the torque converter may not properly lock. Thus, not only are the potential advantages not achieved but clutch durability can be degraded when attempting such a (large negative torque) clutch lock-up.
The disadvantages of prior approaches are overcome by a method for controlling the powertrain of a vehicle, the powertrain having an engine coupled to a transmission via a torque converter having a lock-up clutch. The method comprises determining whether a condition is present in which a negative powertrain output is desired which is less than a predetermined value, and in response to said determination, engaging the clutch before providing said negative powertrain output.
By engaging the clutch before providing the desired negative powertrain output, it is possible to lock the torque converter, even if the torque converter is difficult to lock when transmitting large negative torques.
An advantage of the present invention is that, if desired, it is possible to control negative powertrain torque to a desired level, and even provide a maximum level.
Another advantage of the present invention is that by locking the torque converter in the negative powertrain output region, it is possible, if desired, to provide a smooth transition through the zero torque point and thus minimize potentially degrading effects of transmission gear lash on drivability.
Also, it is important to note that there are several ways in which to determine whether a condition is present in which a negative powertrain output is desired which is less than a predetermined value. For example, this can be done directly from a desired torque value calculated from a driver-actuated signal. Alternatively, it can be determined from a desired engine torque calculated from an engine controller. Or, it can be done simply from certain circumstances known to be those in which negative powertrain output will surely become less than a predetermined value.